1. Field of the Invention
The present invention relates to a torsional damper comprising a pulley hub fixed to a rotary shaft such as a crank shaft, drive shaft, propeller shaft or the like, and a damper mass fixed, by means of a damping rubber, in a cavity formed in the front portion of the pulley hub.
2. Description of the Prior Art
Various types of torsional dampers have so far been proposed, including a typical one which comprises a pulley hub 100 fixed to a rotary shaft such as crank shaft of an engine or the like, and a damper mass 103 fixed, by means of a damping rubber 102, in a cavity formed inside of a cylindrical portion 101 of the pulley hub 100 as shown in FIG. 1. The rotary shaft is installed in a fixing hole 104 in the pulley hub 100. Further, in this conventional torsional damper, a first steel cylindrical damper mass 105 is disposed outside of, concentrically with, and with a radial spacing from, the cylindrical portion 101 of the pulley hub 100, and a damping rubber 106 is interposed between the cylindrical portion 101 and the first damper mass 105. There is interposed between the damping rubber 106 and the cylindrical portion 101 a metallic sleeve 107 press-fitted on the outer circumference of the cylindrical portion 101. The first damper mass 105 absorbs mainly the torsional vibration of the rotary shaft, while the damper mass 103 absorbs mainly the bending vibration of the rotary shaft (a vibration perpendicular to the rotary shaft). A torsional damper of this type is called a "dual type". Furthermore, there is provided an annular retainer 108, of which a bend 109 is fitted on the inner circumference of the cylindrical portion 101, to prevent the damping rubber 102 coupling the damper mass 103 with the pulley hub 100 from being damaged and also the damper mass 103 from falling off the pulley hub 100 and dropping out of the cavity.
As mentioned above, the retainer 108 provided in the conventional torsional damper as a means of preventing the damper mass 103 from falling off the pulley hub 100 has the bend 109 thereof fitted on the inner circumference of the cylindrical portion 101. Hence, the inner circumference of the cylindrical portion 101 must be worked with a high dimensional precision in a large thereof, and also the retainer 108 be worked similarly. For a precision fitting of the retainer 108 into the cylindrical portion 101, the inner circumference of the cylindrical portion 101 and the bend 109 of the retainer 108 must be ground over the large fitting area thereof, which results in increased manufacturing costs. Also, since the retainer 108 is fixed to the cylindrical portion 101 of the pulley hub 100, the pulley hub 100 has an increased weight, possibly resulting in a reduced attenuation of vibration. Furthermore, when the damper mass 103 falls off the pulley hub 100, the cavity in the cylindrical portion 101 and the portion of the pulley hub 100 installed on the rotary shaft will possibly be broken.